A xe2x80x9chybridxe2x80x9d circuit consisting of a substrate with various thick film structures thereon that are interconnected with a plurality of ICs continues to be an attractive technique for creating functionally complex high frequency assemblies from xe2x80x9ccomponentxe2x80x9d ICs. It is often the case that it is necessary or very desirable to use transmission lines to interconnect these ICs, or in connecting them to an external environment. We are particularly interested in the case when the transmission line is of the encapsulated microstrip type described in the incorporated Patent. By the term xe2x80x9cencapsulatedxe2x80x9d that Patent means that the transmission line, which in their example is what would otherwise be called a microstrip, is fully shielded, with a ground completely surrounding the center conductor. It is not exactly what we would ordinarily term a xe2x80x9ccoaxialxe2x80x9d transmission line, since its cross section does not exhibit symmetry about an axis. Nevertheless, we shall find it appropriate and convenient to call it (the xe2x80x98encapsulatedxe2x80x99 transmission line of the ""730 B1 Patent) a xe2x80x98quasi-coaxialxe2x80x99 transmission line, which, it should be noted, is pretty small (perhaps 0.050xe2x80x3 wide by 0.010xe2x80x3 or 0.015xe2x80x3 high). For our present purposes, the most noteworthy thing about the quasi-coaxial transmission line is that it is fabricated on what we may call the component side of the substrate, and that the substrate itself is not used as the dielectric medium for that quasi-coaxial transmission line.
Furthermore, while we bear no ill will toward transmission lines, quasi-coaxial or otherwise, our present purpose is more in keeping with the use of what one would ordinarily call a xe2x80x9cshielded cable.xe2x80x9d In many instances, of course, what distinguishes the two cases is not the cable (they might be identical), but the intended purpose. A power supply voltage or a bias voltage does not need to be delivered via a transmission line, but may well need to be delivered by a shielded conductor, or cable. And, an output microwave carrier should not be conveyed by just any old shielded audio cable, but by a suitable transmission line. Our point is that the suitable transmission line might do both, and that even a suitably shielded coaxial conductor structure might be mistaken for a transmission line (and it would not matter, so long as it was indeed xe2x80x9csuitablexe2x80x9d). Hence, we shall talk less about quasi-coaxial transmission lines than about quasi-coaxial conductors, which might serve either the purpose of a transmission line or the purpose of a shielded connection.
So now we have hybrids with quasi-coaxial conductors to interconnect things. It is true that past hybrids have had ICs and various interconnecting transmission lines to interconnect them. But prior art strip lines and microstrips have heretofore been fabricated using the substrate itself as the dielectric material. A consequence of this is that all the things that one might like to connect to the ground plane are on one side of the substrate, while the ground plane is on the other. For some purposes this is not a problem; through holes can be arranged. But it is inconsistent with other purposes, such as using a portion of the ground plane as one plate in a substantial capacitance. The dielectric constant of the substrate is too low, and it is too thick, to allow fabrication of, say, decent integrated decoupling capacitors that use the substrate as the intervening dielectric.
Unfortunately, just because ICs are small does not mean that they do not draw lots of current. They can, and often do. Unless it is prevented, such effects as instability and oscillation can result, as well as the noise induced by transients. This means that the power supplies and associated bias and other control voltages need to be bypassed, or sometimes filtered. In prior art situations where the ground plane was on the back side of the substrate that meant that a through hole was needed to allow a connection between the bypassing capacitor (which is almost certainly a piece part component) and the signal line being bypassed. This has a definite expense and is bothersome from a production viewpoint, and introduces (an undesirable) extra inductance in series with the bypass. It would be desirable if there were a less expensive, more convenient and more effective way to perform the necessary bypassing. What to do?.
A hybrid circuit having a quasi-coaxial transmission line, or fully shielded conductor, and incorporating a ground plane on the component side of a substrate can bypass and/or filter a power supply voltage or control signal using integrated thick film components and without through holes or vias. A thin pad of suitable dielectric material may be printed onto the ground plane and then have a layer of metal deposited on its top surface, forming a bypass capacitor. The bypass capacitor can be located very near where it is needed, and only a very short conductor is required to connect the metallic top of the bypass capacitor to the location to be bypassed. Since the ground plane is on the component side of the substrate, the short connecting conductor does not go from one side of the substrate to the other, and the thickness and low dielectric constant of the substrate do not compromise the value of the bypass capacitor. Thick film resistors can be included to form filters, and surface mount resistors and capacitors can be used as well, again because the ground plane is on the component side of the substrate. The conductor carrying the signal to be bypassed or filtered to the location where it is needed can be a quasi-coaxial transmission line, even if the signal it carries is not an RF signal needing a controlled impedance path, and is instead merely used as a fully shielded conductor to reduce the effects of stray coupling.